1. Field of the Invention
The invention relates generally to a power transfer mechanism adapted to be incorporated in a multiple path drive system, for example, in a four-wheel drive vehicle. More particularly, it relates to a torque transfer case adapted to receive torque from a prime mover and to provide torque for one drive axle where two-wheel drive is adequate, and to provide torque for both drive axles where four-wheel drive is required.
2. Description of the Prior Art
It is a well known fact that the traction of vehicles can greatly be improved by applying a driving torque to at least two axles of the vehicle at the same time. However, if all the four wheels were driven by the engine, the tire slip would increase when the vehicle makes a turn. Since, during curving, the front wheels must move on radii greater than those of the rear wheels, the revolution speeds of the front wheels are faster than those of the rear wheels. The tires cannot tolerate these adverse conditions for a long period of time, and the undesirable strains that develop in the driving members increase the fuel consumption.
To solve this problem characteristic of four-wheel drive, it is conventionally practiced that a third differential gear is provided in addition to the front- and rear-wheel differential gears to rotate the front and rear drive shafts which in turn drive the front and rear differential gears, respectively, so that when the vehicle changes its direction or when there is a difference between the effective tire radii, only a specific wheel can be driven at high speed.
This kind of differential gear also has a drawback. For example, when one pair of wheels lose traction on a frozen or slippery road surface, they rotate with almost no resistance and this differential gear does not impart driving force to the other pair of wheels. To overcome this problem, the third differential gear is provided with a manual locking device which is actuated to stop the differential action between the two drive shafts and to lock the driven members. However, this kind of locking device is used only for starting and is not necessary when the vehicle is traveling or when the vehicle obtains inertia. In a vehicle whose four wheels are continuously driven, the third differential gear is constantly operated even when the vehicle is traveling on a paved road or when four-wheel drive is not required. This results in increased fuel consumption.